Is Iodine Safe with Levothyroxine?
Free · No account · based on published sources including NIH ODS, FDA drug labels, and peer-reviewed literature
Avoid high-dose iodine supplements (above 1,100 mcg/day) while on levothyroxine therapy. Discuss any kelp, seaweed, or iodine-containing supplement use with your healthcare provider.
Why this interaction occurs
Excess iodine can paradoxically inhibit thyroid hormone synthesis (Wolff-Chaikoff effect) or trigger hyperthyroidism in susceptible individuals. In patients on levothyroxine, high iodine intake can unpredictably alter TSH levels and make thyroid management more difficult.
Evidence
Evidence level: strong.
Reported safety data
The FDA’s adverse event reporting system (FAERS) has received 302,514 reports mentioning Levothyroxine. This reflects reported cases involving the medication in general — not confirmed causation, and not specific to the Levothyroxine + supplement combination on this page.
FAERS reports are voluntary and cannot prove that a drug caused an event. Do not use report counts to assess the safety of this specific drug-supplement pair.
Frequently asked questions
Related interactions
All Iodine interactions → · All Levothyroxine interactions →
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Educational only — not medical advice.
Always confirm with your doctor or pharmacist.